<p>Non-proportional loading has a significant effect on multiaxial fatigue life. To address this phenomenon, several non-proportionality parameters have been proposed in fatigue research. Analogous parameters are also employed in material plasticity models, such as the Tanaka parameter, though they are based on different principles. To date, however, the Tanaka parameter has not been applied in fatigue life analysis. This study presents a preliminary adaptation of the Tanaka parameter for multiaxial fatigue life prediction models. The approach was evaluated using five fatigue damage parameters: Crossland, Findley, Matake, Papadopoulos, and Papuga–Růžička that do not account for non-proportionality. The evaluation covers five materials, MS1, SM45C, X2CrNiMo17-12-2, Cu-ETP, and S460N, selected for their different sensitivities to non-proportional loading. Incorporating the modified Tanaka parameter substantially reduced prediction errors under out-of-phase and asynchronous loading by 19–77%, achieving accuracy comparable to that obtained under proportional loading conditions. These findings indicate that the proposed approach holds promise for extension to other loading types in future studies.</p>